Piezoelectric material is advantageously employed as the actuator in certain active mechanical vibration control apparatus because of its ability to generate substantial countervailing forces with relatively little mass. Additionally, using a piezoelectric element as the actuator makes electronically controlled damping of the overall system feasible and relatively simple when compared to alternative active control methods.
The typical electronic control arrangement includes sensors for detecting the frequency and amplitude of undesired mechanical vibrations occurring on a surface or in an element; and control circuitry responsive to the sensed information for driving the power amplifier of the piezoelectric device. Additionally, the prior art suggests use of a shunt network disposed across the piezoelectric device electrodes. The shunt circuit can in theory substantially cancel the capacitance of the piezoelectric device, with the result that the mechanical damping provided by the device may be increased.
If the mechanical damping required is limited to relatively narrow frequency bands, passive control circuits can generate the requisite narrowband capacitive cancellation using for example L-C resonance circuits. However, realizing a practical active controller circuit in conjunction with capacitance cancellation for broadband vibration damping is more difficult, due to the complexity of the time-spatial distributed nature of the total piezoelectric device-mechanical system. The damping performance of systems suggested to date has been sub-optimal where the vibrations are broadband.
A prime example of the unrealized potential for active damping circuits augmented by a negative capacitance shunt is the boring bar machine tool. When machining stiff or thick-walled workpieces, chatter tends to occur at the bar's first resonant frequency. Embedded piezoelectric reaction mass actuators have been proposed, but none have achieved the needed degree of broadband damping which would allow for precise, uniform cutting.